1. Field of the Invention
The present invention relates to a sensor apparatus for detecting whether a part or a whole of the human body or the like has accessed or entered a predetermined area of a machine concerned. Further, the present invention relates to a safety apparatus implemented by making use of the sensor apparatus for In ensuring protection for the human body or a part thereof approaching a risky area or region of the machine. More particularly, the present invention is concerned with an apparatus for monitoring normality of operation of the sensor apparatuses.
2. Description of Related Art
Among the machines such as exemplified by machine tools, processing/finishing machines, inspection apparatuses, transport mechanisms and the like, there exist such type of machine in which a driving mechanism therefor is exposed to the exterior or easily accessible from the exterior. By way of example, in a rolling mill or machine designed for manufacturing a thin specimen from a plastic chip, such a structure is adopted that the chip is thrown into a gap between a pair of reduction rolls manually by a worker or operator. Consequently, there may undesirably arise such situation that a hand of the operator contacts inadvertently the reduction roll upon charging of the chip into the gap between the rolls.
Under the circumstances, the machine of this sort is ordinarily equipped with a safety apparatus for ensuring safety for the operator. To this end, a sensor apparatus for detecting approach or access of a part of the human body or the like is made use of in the safety apparatus for stopping a driving system for the machine when such access entails danger for the human body.
As the sensor apparatus for the safety apparatus known heretofore, there can be mentioned those disclosed, for example, in Japanese Patent Application Laid-Open Publication Nos. 216692/1988 and 317987/1989 (JP-A-63-216692 and JP-A-1-317987). The sensor apparatus disclosed in these publications is comprised of a pair of electrode plates disposed in opposition to each other.
In the sensor apparatuses mentioned above, change of the capacitance making appearance between the electrode plates is monitored for thereby detecting whether a part of the human body enters or moves through and across the space defined between the electrode plates. In this conjunction, it is noted that the human body has a dielectric constant or permittivity which is considerably larger than that of the air. By taking advantage of such difference in the permittivity between the human body and the air, a capacitance type sensor apparatus is implemented on the basis of the fact that a significant change takes place in the capacitance effective between the electrode plates when a part of the human body is positioned between the electrode plates.
The safety apparatus implemented by adopting the capacitance type sensor mentioned above is indispensably required in order to ensure the safety for the worker or operator. However, such safety apparatus may suffer not a few troubles such as disconnection or breakage of electric conductors, injury and drop-off of electrodes, interruption of electric power supply and the like. Besides, the safety apparatus may undergo adverse influence of the environmental conditions such as change of the ambient electromagnetic field, abnormal rise of the ambient temperature and the like. In that case, abnormal operation of the safety apparatus can take place due to erroneous operation of the capacitance type sensor apparatus, necessitating thus appropriate measures such as shutdown of operation of the driving mechanism for the machine concerned to be taken in order to evade unforecastable accidents. Since occurrence of such troubles and the erroneous operation can not be predicted, operation of the capacitance type sensor apparatus and hence that of the safety apparatus have to be monitored constantly and continuously.
In the light of the state of the art described above, it is an object of the present invention to provide a sensor apparatus and a safety apparatus which are capable of monitoring constantly whether the sensor apparatus is operating normally, to thereby ensure an enhanced safety.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to an aspect of the present invention a sensor apparatus for a machine grounded electrically, which apparatus includes at least one electrode plate disposed relative to the machine with a predetermined space therefrom, an auxiliary electrode disposed in opposition to a portion of the electrode plate, a capacitance change detecting circuit for detecting whether a predetermined dielectric exists or not within the predetermined space on the basis of change of capacitance effective between the electrode plate and the machine, and a monitoring unit for monitoring normality of dielectric detecting operation of the sensor apparatus by electrically connecting periodically the auxiliary electrode plate to the capacitance change detecting circuit.
According to another aspect of the present invention, there is provided a safety apparatus for detecting entrance of at least a part of human body in a predetermined area of a machine grounded electrically for activating safety securing measures. The safety apparatus includes at least one electrode plate disposed relative to the machine with the predetermined area being interposed between the machine and the electrode plate, an auxiliary electrode disposed in opposition to a portion of the electrode plate, a capacitance change detecting circuit for detecting whether a part of the human body exists or not in the predetermined area on the basis of change of capacitance between the electrode plate and the machine, and a monitoring unit for monitoring normality of human body detecting operation of the sensor apparatus by electrically connecting periodically the auxiliary electrode to the capacitance change detecting circuit.
With the structures of the sensor apparatus and the safety apparatus described above, it is possible to monitor constantly with the aid of the auxiliary electrode whether the operation for detecting presence of a dielectric such as the human body or the like is being carried out normally. By virtue of this feature, occurrence of trouble in the sensor/safety apparatus can instantaneously be detected, whereupon proper safety measures such as emergency shutdown of operation of the machine equipped with the sensor/safety apparatus can be validated. Thus, the safety enhanced significantly can be ensured. Besides, because the sensor apparatus and the safety apparatus according to the present invention are implemented as based on the scheme of monitoring the normality of operations of the apparatus by detecting change of electrostatic capacitance, erroneous operation of the sensor apparatus due to variations of the environmental conditions such as the environmental electromagnetic field and the ambient temperature can instantaneously be detected in terms of change of the capacitance, allowing thus the proper measures to be taken immediately, advantageously for ensuring the safety with high reliability.
In a preferred mode for carrying out the invention, the sensor apparatus should further include a guard plate disposed at a rear side of the electrode plate and a shielding plate disposed at a rear side of the guard plate. Owing to such arrangement, no electric field is formed on or along the rear surface of the electrode plate. Thus, the detecting area can be limited to the space defined between the electrode plates. Thus, the possibility of erroneous detection is mitigated or suppressed, whereby the safety apparatus can be operated only within the prescribed range with high reliability, to great advantage.
According to yet another aspect of the present invention, there is provided a sensor apparatus which includes a pair of electrode plates disposed in opposition to each other with a predetermined space being interposed therebetween, an auxiliary electrode disposed in opposition to a portion of either one of the electrode plates, a capacitance change detecting circuit for detecting whether a predetermined dielectric exists or not within the space on the basis of change of capacitance effective between the pair of electrode plates, and a monitoring unit for monitoring normality of dielectric detecting operation of the sensor apparatus by electrically connecting periodically the auxiliary electrode to the capacitance change detecting circuit.
Furthermore, according to still another aspect of the present invention, there is provided a safety apparatus for detecting entrance of at least a part of human body in a predetermined area of a machine grounded electrically, to thereby activate safety securing measures upon detection of the human body. The safety apparatus includes a pair of electrode plates disposed in opposition to each other with the above-mentioned area being interposed between the pair of electrode plates, an auxiliary electrode disposed in opposition to a portion of either one of the electrode plates, a capacitance change detecting circuit for detecting whether at least a part of the human body exists or not in the area mentioned above on the basis of change of capacitance effective between the pair of electrode plates, and a monitoring unit for monitoring normality of human body detecting operation of the sensor apparatus by electrically connecting periodically the auxiliary electrode to the capacitance change detecting circuit.
With the structures of the sensor apparatus and safety apparatus described above, normality of the operation for detecting presence of the human body or a part thereof in the risky area of the machine concerned is monitored periodically with the aid of the auxiliary electrode plate. Thus, occurrence of troubles can instantaneously be detected as well, whereby the safety can further be enhanced.
In a preferred mode for carrying out the present invention, the sensor apparatus may further include a pair of shielding plates grounded electrically and disposed, respectively, in opposition to the pair of electrode plates at sides opposite to the space defined between the electrode plates. By virtue of such arrangement, no electromagnetic field is formed on and along the rear surfaces of the electrode plates owing to the provision of the shielding plates, which contributes to prevention of erroneous detection more positively, to another great advantage.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.